A. Field of the Invention
This invention relates to the manufacture of a catalyst. In one embodiment, it relates to an improved method for stabilizing a finely divided reduced metal catalyst with respect to atmospheric air. In another embodiment, it relates to a method for stabilizing pyrophoric reduced metal catalysts against atmospheric oxidation without loss of metal surface area.
Finely divided, reduced metal catalysts prepared in accordance with prior art procedures are subject to the disadvantage that they are pyrophoric, i.e. when exposed to atmospheric air they tend to oxidize very rapidly. Such a rapid oxidation produces considerable heat and may heat the catalyst to excessively high temperatures. It is understandable both because it makes the catalyst hazardous to handle and because it reduces or destroys the catalytic properties of the catalyst that such action is to be avoided. Hence, it is the usual practice to stabilize the catalyst with respect to atmospheric air following the reduction step.
B. Description of the Prior Art
In accordance with one previously proposed process, described in U.S. Pat. No. 2,677,669 which issued on May 1, 1954, upon completion of the reduction step, nitrogen was introduced into the circulating system and hydrogen vented continuously to cause the nitrogen to replace the hydrogen in the circulating system. At the same time, the bed was cooled to a temperature of around 50.degree. F. and carbon dioxide was introduced into the circulating gas to replace the nitrogen and the nitrogen subsequently vented. When the nitrogen had been replaced by carbon dioxide, elementary oxygen was added to the circulating system in an amount sufficient to produce an oxygen concentration of about 0.1% by volume and circulation of the oxygen-containing gas through the catalyst bed was continued. The bed was provided with a large number of thermocouples to permit observation of temperatures throughout the bed so that "hot spots" could be rapidly and effectively detected. Circulation of the carbon dioxide containing 0.1% oxygen was continued until temperature conditions within the bed were stabilized at 50.degree. F. The oxygen concentration of the circulating gas was then increased in steps by further additions of elementary oxygen and the gas circulating under each oxygen condition until stable temperature conditions were again obtained within the bed. When the bed became stabilized to oxygen concentrations of about 3% at 50.degree. F., it was warmed to 95.degree. F. and atmospheric air was continuously vented to cause the air to replace the carbon dioxide atmosphere. The air was added at a relatively slow rate, i.e. such a rate requires at least six hours for complete replacement of the CO.sub.2 by the air. At the end of this period, it was found that the catalyst was completely stable to atmospheric air and could be packaged or stored as desired without danger of spontaneous overheating.
In another patent, U.S. Pat. No. 2,677,668, there is described a method of stabilizing with respect to atmospheric oxidation, finely divided pyrophoric metal catalysts comprising the steps of exposing said catalyst, after reduction, to an atmosphere of carbon dioxide about room temperature, introducing into said atmosphere, approximately 0.1% by volume of oxygen, feeding oxygen to said atmosphere over a period of time till said catalyst is stable to atmospheric air, and interrupting the addition of oxygen to said atmosphere periodically to prevent the temperature of said catalyst from rising above about 125.degree. F.
Other prior art describing methods for stabilizing finely divided pyrophoric reduced metal catalyst with high metal surface area but less relevant to the method described in the subject application are:
U.S. Pat. No. 2,495,497 describes a process for stabilizing a reduced metal catalyst by wetting, followed by exposure to air until any amount of the liquid has evaporated. Steam or water are disclosed as effective stabilizers. If these stabilizers are used, drying in nitrogen or other inert gas at moderate temperatures restores full catalyst activity;
U.S. Pat. No. 2,565,347 discloses the stabilization of pyrophoric metal catalysts under fluidized conditions. The catalyst is prepared by reducing a finely divided heavy metal oxide, hydroxide or carbonate with a mixture of hydrogen and inert gas, cooling the reduced metal to below 60.degree. C. by a stream of inert gas, and then superficially oxidizing it at below 60.degree. C. with a mixture of oxygen or air and inert gas;
U.S. Pat. No. 3,033,802 which describes a method for stabilizing reduced metal catalysts in which the catalyst is entrained in a carrier stream of non-oxidizing gas and controlled amounts of oxygen-containing gas are added to oxidize the absorbed hydrogen to water vapor;
U.S. Pat. No. 3,838,066 which describes a method for stabilizing pyrophoric materials in a catalyst bed comprising the steps of purging the bed with an inert gas at 100.degree.-500.degree. F. and 15-5000 psig and adding a controlled amount of oxygen-containing gas to the inert gas to oxidize the pyrophoric materials.
East German Pat. No. 14,989 which describes a process whereby hydrogenation catalysts on an alumina carrier are heated at 400.degree.-600.degree. C. for about 24 hours in an oxidizing atmosphere containing O.sub.2, H.sub.2, CO.sub.2 and H.sub.2 O;
Japanese Pat. No. 4,325 (1958) which shows that the stability of reduced nickel is increased by heating in an inert atmosphere, e.g., CO.sub.2 or N.sub.2 ;
Japanese Pat. No. 6,365 (1960) which discloses a process wherein Raney nickel or copper directly obtained by a NaOH solution is washed with water and then with an alcohol while passing CO.sub.2 through the catalyst to remove oxygen completely. The catalyst is then treated in CO.sub.2 at 3-5 mm and at 70.degree. C. It is alleged that this catalyst has improved stability in air;
Yamanaka and Takagi, J. Sci. Research Inst. (Tokyo), 51, 168-9 (1957) disclose a process for the stabilization of nickel catalyst with CO.sub.2 as the inert gas at elevated temperatures followed by cooling to ordinary temperatures in the CO.sub.2 atmosphere and retained in the CO.sub.2 atmosphere. It is disclosed that these stabilized hydrogenation catalysts have improved efficiency and retain their activity for 1 year if kept under airtight seal and for 5 hours if left exposed.
Popowicz et al., Prsemysl Chem., 44 (6), 305-8 (1965) (Poland) also disclose a process for the stabilization of nickel catalysts on an Al.sub.2 O.sub.3 carrier by treating the reduced catalyst at 30.degree. with N.sub.2 containing a small amount of O.sub.2 (about 0.15% by volume) to remove the adsorbed hydrogen. It is disclosed that both the amount of hydrogen present in the catalyst and the activity of the latter after reactivation depend on the temperature and the reactivation time.
Schmitt, J. Fire Flammability, 2 (April), 157-72 (1971) has provided a literature review on pyrophoric materials which includes fundamental information on various types of pyrophoric materials so as to indicate the various factors that affect pyrophoricity.